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26                                HYDROCHLORIC ACID

bath, should leave no appreciable residue, and 2 c.c., diluted with 20 c.c.
of water and rendered alkaline with ammonia, should undergo no change
with ammonium sulphide.

5.  Acrolein,   Allyl   Alcohol,   Empyreumatic    Products.—These
are recognised by the odour, after neutralisation of the acid with sodium

hydroxide.

6.  Determination of the Formic Acid.—This may be effected by
titration with normal alkali solution in presence of plienolphtlialein (i c.c.
N-alkali = 0-046 gram of ,1-1^:0,) and controlled by the specific gravity
of the sample.   A marked difference between the two results indicates
extraneous acids or salts.

The formic acicl may be determined directly as follows : 12 grams of
the acicl are neutralised exactly with sodium hydroxide, treated with an
excess of mercuric chloride solution and heated on a, steam-bath. The
mercurous chloride thus formed is then collected on a tared filter, washed,
dried and weighed (i gram Hg2Cla = 0-0977 gram of II.,('(),).

For determining the formic acid in formates, the latter are decomposed
by phosphoric acid and distilled, the formic acid in the distillate being
determined as above.

Almost pure formic acicl, containing only traces of hydrochloric, acid and
sodium sulphate, may now be purchased. Sonic of the less pure forms contain
up to 2% of hydrochloric acid. The mixtures with, acetic, acid (aoelargol) have
been already mentioned.

HYDROCHLORIC   ACID

HC1 - 3()'4f> (3<>'5)

The crude acid of commerce is more, or less yellow, I) about 1-1,8, con-
taining about 35%HC1. The pure acidis colourless and the most: concen-
trated has D = ri9—r*o with a content of ;i7™j()% IK-I.

The crude acid may contain sulphuric, acid, chlorine, bromine, iodine,
arsenic, iron, lime, alkalies, and organic matter, The pure acid may contain
the same impurities but naturally in smaller proportions.

Its analysis includes the following :

1.  Fixed Residue.—50 c.c. are evaporated in a platinum dish and
the residue, if appreciable1., weighed,

2.  Sulphuric and Sulphurous Acids.--lo c.c,, diluted with 50 c.c.
of water, are treated with barium, chloride ;   no turbidity should appear
even after 12 hours.   Addition of a few drops of chlorine water to the,
liquid, filtered if necessary, will cause turbidity if sulphurous acid is present.

Sulphurous acid may also be de.tected by treating the acid with a piece
of pure zinc and testing the gas evolved for hydrogen sulphide by means
of lead acetate paper.

The sulphuric acid may be determined quantitatively by precipitation
as barium sulphate (i gram BaS04 — 0-34335 gram S()3).

3.  Arsenic.—i c.c. of the acid is treated with 5 c.c. of Bettendorfs
reagent (see note on p. 18) and note made if the liquid colours within an
hour.en treated with excess of pure formic